The invention is directed towards the field of light emission particularly towards providing high quality reflective surfaces to both sides of an AlxGayInzN device,
A vertical cavity optoelectronic structure consists of an active region that is formed by light emitting layer interposing confining layers that may be doped, un-doped, or contain a p-n junction. The structure also contains at least one reflective mirror that forms a Fabry-Perot cavity in the direction normal to the light emitting layers. Fabricating a vertical cavity optoelectronic structure in the GaN/AlxGayInzN/AlxGa1xe2x88x92xN (where x+y+z=1 in AlxGayInzN and where xxe2x89xa61 in AlxGa1xe2x88x92xN) material systems poses challenges that set it apart from other III-V material systems. It is difficult to grow AlxGayInzN structures with high optical quality. Current spreading is a major concern for AlxGayInzN devices. Lateral current spreading in the p-type material is xcx9c30 times less than that in the n-type material. Furthermore, the low thermal conductivity of many of the substrates adds complexity to the device design, since the devices should be mounted junction down for optimal heat sinking.
One vertical cavity optoelectronic structure, e.g. a vertical cavity surface emitting laser (VCSEL), requires high quality mirrors, e.g. 99.5% reflectivity. One method to achieve high quality mirrors is through semiconductor growth techniques. To reach the high reflectivity required of distributed Bragg reflectors (DBRs) suitable for VCSELs ( greater than 99%), there are serious material issues for the growth of semiconductor AlxGayInzN DBRs, including cracking and electrical conductivity. These mirrors require many periods/layers of alternating indium aluminum gallium nitride compositions (AlxGayInzN/Alx.Gay.Inz.N). Dielectric DBRs (D-DBR), in contrast to semiconductor DBRs, are relatively straightforward to make with reflectivities in excess of 99% in the spectral range spanned by the AlxGayInzN system. These mirrors are typically deposited by evaporation or sputter techniques, but MBE (molecular beam epitaxial) and MOCVD (metal-organic chemical vapor deposition) can also be employed. However, only one side of the active region can be accessed for D-DBR deposition unless the growth substrate is removed. Producing an AlxGayInzN vertical cavity optoelectronic structure would be significantly easier if it was possible to bond and/or deposit D-DBRs on both sides of a AlxGayInzN active region.
Wafer bonding can be divided into two basic categories: direct wafer bonding, and metallic wafer bonding. In direct wafer bonding, the two wafers are fused together via mass transport at the bonding interface. Direct wafer bonding can be performed between any combination of semiconductor, oxide, and dielectric materials. It is usually done at high temperature ( greater than 400xc2x0 C.) and under uniaxial pressure. One suitable direct wafer bonding technique is described by Kish, et al., in U.S. Pat. No. 5,502,316. In metallic wafer bonding, a metallic layer is deposited between the two bonding substrates to cause them to adhere. One example of metallic bonding, disclosed by Yablonovitch, et al. in Applied Physics Letters, vol. 56, pp. 2419-2421, 1990, is flip-chip bonding, a technique used in the micro- and optoelectronics industry to attach a device upside down onto a substrate. Since flip-chip bonding is used to improve the heat sinking of a device, removal of the substrate depends upon the device structure and conventionally the only requirements of the metallic bonding layer are that it be electrically conductive and mechanically robust.
In xe2x80x9cLow threshold, wafer fused long wavelength vertical cavity lasersxe2x80x9d, Applied Physics Letters, Vol. 64, No. 12, 1994, pp1463-1465, Dudley, et al. taught direct wafer bonding of AlAs/GaAs semiconductor DBRs to one side of a vertical cavity structure while in xe2x80x9cRoom-Temperature Continuous-Wave Operation of 1.54-xcexcm Vertical-Cavity Lasers,xe2x80x9d IEEE Photonics Technology Letters, Vol. 7, No. 11, November 1995, Babic, et al. taught direct wafer bonded semiconductor DBRs to both sides of an InGaAsP VCSEL to use the large refractive index variations between AlAs/GaAs. As will be described, wafer bonding D-DBRs to AlxGayInzN is significantly more complicated than semiconductor to semiconductor wafer bonding, and was not known previously in the art.
In xe2x80x9cDielectrically-Bonded Long Wavelength Vertical Cavity Laser on GaAs Substrates Using Strain-Compensated Multiple Quantum Wells,xe2x80x9d IEEE Photonics Technology Letters, Vol. 5, No. 12, December 1994, Chua et al. disclosed AlAs/GaAs semiconductor DBRs attached to an InGaAsP laser by means of a spin-on glass layer. Spin-on glass is not a suitable material for bonding in a VCSEL between the active layers and the DBR because it is difficult to control the precise thickness of spin on glass, and hence the critical layer control needed for a VCSEL cavity is lost. Furthermore, the properties of the glass may be inhomogeneous, causing scattering and other losses in the cavity.
Optical mirror growth of AlxGa1xe2x88x92xN/GaN pairs of semiconductor DBR mirrors with reflectivities adequate for VCSELs, e.g.  greater than 99%, is difficult. Referring to FIG. 1, theoretical calculations of reflectivity suggest that to achieve the required high reflectivity, a high index contrast is required that can only be provided by increasing the Al composition of the low-index AlxGa1xe2x88x92xN layer and/or by including more layer periods (material properties taken from Ambacher et al., MRS Internet Journal of Nitride Semiconductor Research, 2(22) 1997). Either of these approaches introduces serious challenges. If current will be conducted through the DBR layers, it is important that the DBRs be conductive. To be sufficiently conductive, the AlxGa1xe2x88x92xN layer must be adequately doped. Electrical conductivity is insufficient unless the Al composition is reduced to below approximately 50% for Si (n-type) doping and to below approximately 20% for Mg (p-type) doping. However, as shown in FIG. 1, the number of layer periods needed to achieve sufficient reflectivity using lower Al composition layers requires a large total thickness of AlxGa1xe2x88x92xN material, increasing the risk of epitaxial layer cracking (due to the relatively large lattice mismatch between AlN and GaN) and reducing compositional control. Indeed, the Al0.30Ga0.70N/GaN stack of FIG. 1 is already xcx9c2.5 xcexcm thick and is far from sufficiently reflective for a VCSEL. Thus, a high reflectivity DBR based on this layer pair requires a total thickness significantly greater than 2.5 xcexcm and would be difficult to grow reliably given the mismatch between AlN and GaN growth conditions and material properties. Even though the cracking is not as great of an issue if the layers are un-doped, compositional control and the AlN/GaN growth temperatures still pose great challenges to growing high reflectivity DBRs. Hence, even in applications where the DBRs do not have to conduct current, semiconductor mirror stacks with reflectivities  greater than 99% in the AlxGayInzN material system have not been demonstrated. For this reason, dielectric-based DBR mirrors are preferred.
At least one mirror stack, e.g. a dielectric distributed Bragg reflector (D-DBR) or composite D-DBR/semiconductor DBR interposes a AlxGayInzN active region and a host substrate. A wafer bond interface is positioned somewhere between the host substrate and the active region. An optional intermediate bonding layer is adjacent the wafer bond interface to accommodate strain and thermal coefficient mismatch at the wafer bond interface. An optional mirror stack is positioned adjacent the AlxGayInzN active region. Either the host substrate or intermediate bonding layer is selected for compliancy.
One embodiment of the aforementioned invention consists of a device having the wafer bond interface positioned adjacent the AlxGayInzN active region, the AlxGayInzN active region is fabricated on a sacrificial substrate, e.g. Al2O3. The mirror stack attached to a host substrate is direct wafer bonded to the AlxGayInzN active region. Next, the sacrificial substrate is removed. The optional mirror stack is attached to the top of the AlxGayInzN active region. Techniques for attaching include bonding, depositing, and growing. Electrical contacts are added to the n-type and p-type layers.
For an alternate embodiment having the wafer bond interface positioned adjacent the host substrate, the mirror stack is attached on top of the AlxGayInzN active region. If direct wafer bonding is employed, a host substrate, selected to have the proper mechanical properties, is wafer bonded to the mirror stack. Alternatively, metallic bonding may be used to bond the host substrate to the mirror stack. The sacrificial substrate is removed. An optional mirror stack is attached on top of the AlxGayInzN active region. Electrical contacts are added to the n-type and p-type layers. Selection of the host substrate in cases of direct wafer bonding to obtain the desired properties is a critical teaching. Additional embodiments include positioning the wafer bond interface within the DBR.